The present invention relates in general to packages for integrated circuits and, more particularly, it relates to ceramic flat packages. While the invention will be described with reference to square or so-called "quad" or "chip carrier" packages, the principles of same are equally applicable to the well-known dual-in-line or "DIP" configuration. Both square and rectangular frames and packages may be referred to as "quadrilateral."
The growing demand for larger and more complex integrated circuitry, plus design advances allowing packing of more and more circuit elements on only a small area or "real estate" of silicon, have not been matched by corresponding advances in package design. Of these two factors, volume of demand has created the more serious problems, insofar as while automatic or automated die attach and wire bonding machines are available, their application and usage have been limited by package design per se as well as dimensional tolerances that are too great for optimum utilization.
Integrated circuit packages take many forms, but in general they include a substrate which may include a shallow cavity which holds the silicon die or chip, either a metallized and plated lead pattern or a metallic lead frame having inner lead tips surrounding the cavity and leads extending out to the edge of the substrate where the leads (in a frame) are bent in a suitable configuration for electrical connection into a socket, circuit board or whatever.
Plated patterns must also generally be attached to metal leads near the outer substrate edge or to a carrier or plug-in for connection to a circuit.
Lead frames are so named because, as formed by stamping or etching, all the leads for a device are held together by an outer, connecting frame. As the number of leads gets larger lead inner-tips must also be held in place with an internal tie bar, and the intermediate tie bars are often needed. All such frame and tie-bar elements are ultimately removed so that each lead is electrically isolated from its neighbors, but not before the package assembly is complete and the leads are firmly sealed in position.
The operation called die attach secures the die in the bottom of the cavity, and wire bond is the operation of connecting individual contact pads on the die with individual inner lead tips, generally with extremely fine gold or aluminum wire.
Lastly, the cavity is hermetically sealed with a cover. Die attach and sealing may involve metallurgical bonds, glassy bonds or compounds such as plastics and epoxies. Wire bonding must involve a metallurgical bond, generally created by thermocompression or ultrasonic methods. All of the foregoing is generally known in the art, and there are many variations.
The problem that is created with existing package designs and dimensional tolerances is that the die, after attachment in the substrate cavity, is either positioned
with reference to the substrate but not the bond area or the leads, or
with reference to the bond area but not the substrate or leads, or
with reference to the leads but not the bond area or substrate.
The next result is that the contact pads and the lead tips are not in their proper relation to each other, and even wire bonders with pattern recognition capability can make errors or be greatly slowed.
The foregoing lead-tip/contact pad problem may be considered as existing in plan view, e.g. when the package and die are viewed from above. Another problem exists in elevation or side view: existing package designs and assembly procedures allow lead height variation and lack of strict parallelism between the plane of the die and the plane of leads. Automated wire bonders are also sensitive to such variations.